Synchrotron radiation has an important advantage of high flux on even small samples. In particular, the in-situ or ex-situ high-pressure studies are performed almost entirely at synchrotron beamlines. Users of synchrotron beams are aware of possible wavelength instabilities which may arise during the measurements (illustrated e.g. in [1]). The instabilities are due largely to thermomechanical phenomena at primary-beam monochromators. For many purposes, the instabilities can be neglected. The stability requirements are particularly severe in thermal expansion measurements, as the changes of the lattice constants with temperature are extremely small. In such studies we have noticed that different a(T) runs of the given material differ slightly. Using an internal wavelength standard was expected to reduce this effect. The diamond standard was chosen because of its known small thermal expansivity making that the possible effect, due to impurities or to the defect structure of the standard, on the final result is also small. We used a 99.9% pure diamond powder with single crystal grains of about one micrometer size. The low-temperature XRD measurements were carried out at a powder diffractometer (for construction details see [2]) at the B2 bending-magnet beamline using Debye-Scherrer geometry. The closed-circuit He-cryostat ensured good temperature stability and accuracy. The results of thermal expansion coefficient measured for several materials in the 10-300 K range using the diamond standard are found to be reproducible. Moreover, they exhibit a good agreement with the first-principles theoretical calculations.